qemu: disable qemu user emulation on MIPS64 for host variant

qemu: disable qemu user emulation on MIPS64 for host variant

According to target/mips/TODO in the Qemu sources:

MIPS64
------
- Userland emulation (both n32 and n64) not functional.

And indeed, trying to run a mips64n32 binary under qemu user emulation
results in:

Invalid ELF image for this architecture

So we move the BR2_mips64(el) dependency from
BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS to
BR2_PACKAGE_HOST_QEMU_SYSTEM_ARCH_SUPPORTS, so that only the system
emulation is available on mips64, and not the user-mode emulation.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>